The present invention relates to transducers that utilize induced strain to measure acceleration pressure, temperature and other variables, and more particularly to providing temperature and static pressure compensation in such devices.
Resonant transducers have been used for many years to achieve high accuracy measurements. Vibrating transducers have been used in precision accelerometers and pressure sensors. These devices operate on the principle that the natural frequency of vibration (i.e. resonant frequency of an oscillating beam or other member) is a function of the induced strain along the member. More particularly, tensile forces elongating the beam increase its resonant frequency, while forces compressing the beam reduce the natural frequency. The frequency output of resonant gages is readily converted to digital readings reflecting the measured quantity, requiring only a counter and a reference clock for this purpose. Thus, such gages are simple and reliable, and provide a high degree of discrimination while using a relatively simple interface to digital signal processing circuitry.
One particularly effective transducer of this type is a resonant integrated microbeam sensor, for example as disclosed in U.S. patent application Ser. No. 07/937,068, filed Aug. 31, 1992 and now U.S. Pat. No. 5,275,055 entitled "Resonant Gage with Microbeam Driven in Constant Electric Field", and assigned to the assignee of this application. The sensor includes a silicon substrate, a polysilicon flexure beam attached at both ends to the substrate, and a polysilicon rigid cover cooperating with the substrate to enclose the flexure beam within a sealed vacuum chamber. A pair of bias electrodes on opposite sides of the beam create a constant electrical field about the flexure beam. A drive electrode on the flexure beam is selectively charged to oscillate the beam. A piezoresistive element on the flexure beam is used to indicate beam position, and also to provide feedback to the drive oscillator. Thus, the beam tends to oscillate at its natural resonant frequency.
The sensor can be fabricated on a pressure sensor diaphragm or a flexure of an accelerometer, to be elongated or compressed by deflections of the diaphragm or flexure in response to pressure changes and accelerations, respectively. While satisfactory in many of these applications, the sensors are subject to error due to deviations in temperature and in static pressure.
It is known, in connection with resonant sensors, to provide compensation for variations in temperatures and other conditions. For example, U.S. Pat. No. 4,535,638 (EerNisse et al) discloses a resonator transducer system in which a vibratory element such as a quartz crystal is driven to oscillate at two frequencies, both of which vary with changes in applied force and changes in temperature. The frequency outputs are processed by a computer containing predetermined coefficients for correcting as to the temperature effect.
U.S. Pat. No. 4,598,381 (Cucci) discloses a pressure compensated differential pressure sensor. A reference sensor senses a relatively low reference pressure, and a second sensor senses a differential between the lower pressure and a higher second pressure. Outputs of the pressure sensors and a temperature sensor are provided to a computer, programmed to correct for temperature effects. The computer includes an analog to digital converter receiving the temperature sensor output.
In U.S. Pat. No. 4,765,188 (Krechmery et al.), a pressure transducer includes a diaphragm with several piezoresistor strain gages for sensing pressure. A temperature dependant resistor also is formed on the diaphragm. The output of the temperature sensitive resistor is converted to a digital signal provided to a programmable read only memory (Prom). The Prom stores correction data to provide temperature compensation.
While the above approaches are workable, they require storage of compensation data, and frequently require analog to digital conversion, adding on to complexity of sensing and compensation circuitry and thus increasing the difficulty of semiconductor device fabrication.
Therefore, it is an object of the present invention to provide a resonant sensing device in which sensors provided for compensation generate digital outputs, eliminating the need for analog to digital signal conversion.
Another object is to provide, in a single measuring device, the combination of a primary resonant sensor and at least one secondary resonant sensor for compensation.
Yet another object is to provide a measuring device in which one or more secondary resonant sensors used for compensation are substantially identical in size and internal strain, to provide for simple and reliable compensation that does not require storage of multiple compensation values in computer memory.